Electron beam devices, in particular a scanning electron microscope (also referred to as SEM below) and/or a transmission electron microscope (also referred to as TEM below), are used to examine objects (also referred to as samples) in order to obtain knowledge in respect of the properties and behavior of the objects under certain conditions.
In an SEM, an electron beam (also referred to as primary electron beam below) is generated by means of a beam generator and is focused on an object to be examined by means of a beam guiding system. An objective lens is used for focusing purposes. The primary electron beam is guided over a surface of the object to be examined by means of a deflection device. This is also referred to as scanning. The area scanned by the primary electron beam is also referred to as scanning region. Here, the electrons of the primary electron beam interact with the object to be examined. Interaction particles and/or interaction radiation result as a consequence of the interaction. By way of example, the interaction particles are electrons. In particular, electrons are emitted by the object—the so-called secondary electrons—and electrons of the primary electron beam are scattered back—the so-called backscattered electrons. The interaction particles form the so-called secondary particle beam and are detected by at least one particle detector. The particle detector generates detection signals which are used to generate an image of the object. An image of the object to be examined is thus obtained. By way of example, the interaction radiation is X-ray radiation or cathodoluminescence light. At least one radiation detector is used to detect the interaction radiation.
In the case of a TEM, a primary electron beam is likewise generated by means of a beam generator and directed onto an object to be examined by means of a beam guiding system. The primary electron beam passes through the object to be examined. When the primary electron beam passes through the object to be examined, the electrons of the primary electron beam interact with the material of the object to be examined. The electrons passing through the object to be examined are imaged onto a luminescent screen or onto a detector—for example in the form of a camera—by a system comprising an objective. By way of example, the aforementioned system additionally also comprises a projection lens. Here, imaging may also take place in the scanning mode of a TEM. Such a TEM is referred to as STEM. Additionally, provision may be made for detecting electrons scattered back at the object to be examined and/or secondary electrons emitted by the object to be examined, by means of at least one further detector in order to image the object to be examined.
Combining the functions of an STEM and an SEM in a single particle beam device is known. It is therefore possible to carry out examinations of objects with an SEM function and/or with an STEM function using this particle beam device.
Moreover, a particle beam device in the form of an ion beam column is known. Ions used for processing an object are generated using an ion beam generator arranged in the ion beam column. By way of example, material of the object is ablated, or material is applied onto the object during the processing. The ions are additionally or alternatively used for imaging.
Furthermore, the prior art has disclosed the practice of analyzing and/or processing an object in a particle beam device using, on the one hand, electrons and, on the other hand, ions. By way of example, an electron beam column having the function of an SEM is arranged at the particle beam device. Additionally, an ion beam column, which has been explained further above, is arranged at the particle beam device. The electron beam column with the SEM function serves, in particular, for examining further the processed or unprocessed object, but also for processing the object.
A particle beam generator in the form of an electron gun is known from the prior art. The known electron gun comprises an electron source having an electron emission surface. Furthermore, the electron gun comprises a first electrode configured to control the path of electrons emitted from the electron emission surface, a second electrode configured to suppress emissions of electrons from a side surface of the electron source and a third electrode configured to accelerate electrons emitted from the electron source to a final energy.
A further particle beam generator in the form of an ion beam generator is also known from the prior art. The ion beam generator comprises an ion source configured to emit ions, a suppressor electrode configured to suppress the emitted ions from a side surface of the ion source, an extractor electrode configured to extract the ions from the ion source, a first variable voltage supply unit for biasing the extractor electrode with an extractor voltage and a second variable voltage supply unit for biasing the suppressor electrode with a suppressor voltage. The particle beam generator provides an emission current comprising the ions.
When using the known ion beam generator, the emission current may follow a specific behavior being dependent on time due to inherent physical characteristics of the ion beam generator. FIG. 1A shows such specific behavior of the emission current EC. In other words, FIG. 1A shows the emission physiology of the known ion beam generator. The emission current EC decreases after an initial time T0. When the emission current EC reaches a minimum at a time TMIN, the emission current EC increases for times after TMIN until it reaches a maximum at the time TMAX. After the time TMAX, the emission current EC decreases again.
When using the ion beam generator, one is intent on obtaining a more or less constant and specific emission current of the ion beam generator. Typical specific emission currents of the ion beam generator are in the range of 1.8 μA to 2.2 μA. For example, the specific emission current of the ion beam generator is 2 μA (see FIG. 1A). It is known to adjust the suppressor voltage applied to the suppressor electrode such that the specific emission current of the ion beam generator is reached or maintained (see FIGS. 1A and 1B). For example, if the emission current EC increases, the suppressor voltage applied to the suppressor electrode is also increased. However, when the emission current EC decreases, the suppressor voltage applied to the suppressor electrode is also decreased. By increasing or decreasing the suppressor voltage applied to the suppressor electrode, the emission current EC of the ion beam generator is adjusted to the specific emission current, for example 2 μA.
If the emission current decreases and falls below a specific threshold, the suppressor voltage applied to the suppressor electrode is also decreased and might reach a lower threshold value of 0 V, and, therefore, does not influence the emission current anymore (see FIG. 1B). If the suppressor voltage applied to the suppressor electrode does not influence the emission current anymore, this might lead to an exhaustion of the ion source which is unwanted. In other words, the specific emission current decreases until it vanishes. If the specific emission current is not reached or maintained, it is known to adjust the extractor voltage applied to the extractor electrode to a new value of the extractor voltage such that the specific emission current is reached or maintained. At this new value of the extractor voltage, the suppressor may influence the emission current and keep the specific emission current stable. However, the new value of the extractor voltage might differ from the previous value of the extractor voltage by a few hundred Volts. This might result in the necessity to realign the particle beam impinging on the object and, therefore, to readjust the particle beam current and particle beam shape on the object. In other words, the path of the ions in the ion beam column is altered due to the change of the extractor voltage and might not be focused on the object anymore. Therefore, the characteristics of all further beam guiding units, in particular the voltages applied to these beam guiding units, have to be changed also such that the beam of ions is realigned and such that the ions travel on a path through the ion beam column suitable for focusing the ions on the object. The effort of a realignment of the ion beam impinging on the object and, therefore, the readjustment of the ion beam current and the ion beam shape on the object may be high and should be avoided, if possible.
If the emission current increases above a specific threshold, the suppressor voltage applied to the suppressor electrode is also increased and might reach an upper threshold value, for example 2 kV and therefore, is not able to uphold the emission anymore. Accordingly, quality of the particle beam decreases. Moreover, the specific emission current may not be reached or maintained anymore using a specific value of the extractor voltage due to physical characteristics of the ion source.
Further methods and devices for adjusting and/or controlling the emission current of an ion beam generator are also known in the prior art. For example, the emission current may be stabilized by filament current variations or by mechanical arrangements.
With respect to the prior art, we refer to EP 2 264 738 A1, U.S. Pat. Nos. 5,111,053 A, 7,238,952 B2, 5,399,865 A and US 2007/0257200 A1.